This is an excellent question and one that is at the forefront of cosmology research. The short answer is that we do not yet know.
Dark matter particles, being responsive to Newton’s laws of gravity, are certainly allowed to clump just like the ordinary matter that is made of protons, neutrons, and other familiar subatomic particles. Dark matter could, therefore, collapse into objects of any size. In fact, there is strong evidence today for the existence of dark matter “halos” — spherical clumps of dark matter centered around every galaxy that extend 10 times larger than the radius spanned by the galaxy’s stars.
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